1. Field of the Invention
This invention relates to directional microphones. In particular, the invention relates to a directional microphone having a non-symmetrical pickup pattern.
2. Description of the Prior Art
In general, there are two well known types of microphones, i.e., nondirectional (omnidirectional) and directional. Omnidirectional microphones pick up sound from all directions and are used in environments where ambient sound is not a problem. Directional microphones pick up sounds primarily from one direction or area and are particularly used where there are environmental noise problems, where maximum gain-before-feedback is required, or where there are great distances from microphone to sound source.
The cardioid pickup pattern is a well known response pattern for directional microphones, characterized by maximum pickup sensitivity or response for sound coming from the desired direction, known as "on-axis", with diminishing response for sound coming from other directions. For a true cardioid pattern, the response is down, e.g., 6 dB at 90 degrees to on-axis, and has a "null" at 180 degrees to on-axis. There are many known variations of the cardioid pattern, but in general these characteristics occur.
Currently known cardioid and cardioid-family microphones have uniformly symmetric response curves with respect to rotation of the microphone about its longitudinal axis. Any deviation from this uniformity has generally been viewed as an undesirable feature, to be avoided or corrected when present.
In some applications, however, a microphone is not used with nor can it be used with its longitudinal axis aligned with the direction from which the desired sound is coming. For example, a professional singer or radio broadcaster may position a cardioid microphone so that the voice is on-axis, utilizing the maximum pickup sensitivity of the microphone. For a television broadcaster to do so, however, would cause the microphone to be held in an awkward position which appears unnatural or uncomfortable and which prevents an unobstructed camera angle of the person speaking and holding the microphone.
In many other situations, a directional microphone is rigidly mounted in a spaced relationship from a particular sound source, but for some reason, the microphone cannot be aimed at the sound source, such as in an airplane cockpit microphone mounted on a control panel for use in a flight recorder system, also known as a "black box" system.
In each of these situations, the sound source cannot be placed on-axis, and thus the maximum pickup sensitivity of the microphone is not utilized. In fact, maximum pickup is provided in a direction of unwanted sound. Due to the symmetry of known cardioid microphones and the uniformity of symmetry in all planes, off-axis pickup on one side of the microphone gives the same performance as on any other side. This invention relates to the problem of improving off-axis sensitivity of cardioid and cardioid-family microphones when the sound source can be accurately placed in a particular side of the microphone, albeit still off-axis.